What level of protein structure is associated with the $\alpha$ helix and $\beta$-pleated sheet?

• A. Quaternary structure
• B. Tertiary structure
• C. Primary structure
• D. Secondary structure

Answer: (D)

Verified step by step guidance

1. Understand the four levels of protein structure: primary, secondary, tertiary, and quaternary.
2. The primary structure refers to the linear sequence of amino acids in a protein.
3. The secondary structure involves local folding of the polypeptide chain into structures such as alpha helices and beta-pleated sheets, stabilized by hydrogen bonds.
4. The tertiary structure is the overall three-dimensional shape of a single protein molecule, formed by interactions between side chains.
5. The quaternary structure involves the assembly of multiple protein subunits into a larger complex.